Amazon UK? I had an email from them saying the order will be cancelled if it isn't shipped within a few days as it'd been so long since ordering (despite it not being out yet). Then the next day I had "order dispatched".
This was 20th Feb. Pretty sure it doesn't take over 3 weeks to send a screen protector. My guess is the seller has marked it as dispatched to prevent the order being auto cancelled.
That's exactly the case. They sent emails to anyone that had it preordered.